Separation of natural gas mixtures



Jan. 22, 1952 I J 051- 2,583,090

SEPARATION OF NATURAL GAS MIXTURES Filed Dec. 29, 1950 ,5 Sheets-Sheet 1IN VEN TOR.

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Jan. 22, 1952 J. L. COST 2,583,090

' SEPARATION OF NATURAL GAS MIXTURES Filed Dec. 29, 1950 5 Sheets-Sheet2 IN VEN TOR.

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SEPARATION OF NATURAL GAS MIXTURES Filed Dec. 29, 1950 s Sheets-Sheet 3Patented Jan. 22, 1952 SEPARATION OF NATURAL GAS MIXTURES Joe L. Cost,Madeira, Ohio, assignor to Elliott Company, Jeannette, Pa., acorporation of Pennsylvania Application December 29, 1950, Serial No.203,412

9 Claims. 1

This invention relates to the separation of natural gas mixtures byrectification at low temperature, and more particularly, to theseparation of mixtures composed substantially of nitrogen and methane torecover the latter as a gas.

It is among the objects of this invention to provide a method andapparatus for effecting the separation above described, in which therewill be available suilicient refrigeration at low temperatures to assurea high yield of methane product with a minimum expenditure of power inproducing that refrigeration and with a minimum amount of equipment toutilize it.

It is a further object of the invention to separate nitrogen-methanemixtures and recover a gaseous methane product therefrom at asubstantial pressure for delivery to gas distribution mains, or tostorage, with little or no additional compression of that product.

In accordance with this invention, natural gas of the indicatedcomposition is compressed to a very high pressure; and it is thensuccessively cooled to a low temperature, expanded to an intermediatepressure, and rectified at that pressure to separate its containednitrogen and methane. The cold nitrogen so obtained is cooled andpartially liquefied, the liquefied portion providing the liquid refluxnecessary to assure a high degree of rectification and, thereby, a highyield of separated methane, while the unliquefied portion provides thelow temperature refrigeration necessary for such partial liquefaction.That refrigeration is made available by expanding the unliquefiedportion of the separated nitrogen in a plurality of stages tosubstantially atmospheric pressure with the performance of externalwork, and using the cooled nitrogen resulting from each expansion toabsorb heat from the unexpanded nitrogen.

The invention is illustrated in the accompanying drawings, in which Fig.1 shows diagrammatically one embodiment thereof Fig. 2, a modification,in which the unliquefied portion of the separated nitrogen is compressedbetween successive expansions; and Fig. 3, a modification of Fig. 2.

Referring to Fig. 1, natural gas containing, for example, about fortyper cent nitrogen and sixty per cent methane is compressed in amulti-stage compressor I, provided with the usual aftercoolers, to apressure of around 200 atmospheres. The compressed mixture at atemperature 01' about 100 F. is passed through a countercurrent heatexchanger 2 and is there cooled by the outgoing cold separation productsto a temperature of about -125 F. It is further cooled to about -l85 F.and partially liquefied in a reboiler 3 at the bottom of a rectifyingcolumn 4, where the mixture gives up heat to liquid methane surroundingthe reboiler. The mixture is further liquefied and subcooled in a secondheat exchanger 6 by giving up heat to cold separated nitrogen, ashereinafter described. After leaving exchanger 6, the mixture passesthrough a throttle valve 1, where it is expanded to an intermediatepressure of about 130 p. s. i. a. and is thereby cooled to a still lowertemperature. It is then introduced into the rectifying column 4 at anintermediate level and there rectified in the usual manner.

The cold gaseous nitrogen that has been separated from the mixture byrectification leaves the top of the column through a pipe 8 at atemperature of about -245 F. and at the intermediate pressure prevailingin the column. This cold nitrogen passes through a liquefier 9, where itis cooled and partially liquefied, as hereinafter described. Theresulting liquid is separated from the unliquefied portion in aseparator ll, withdrawn from the separator through a pipe I2, andintroduced into the top of the column as liquid reflux at a temperatureof about 253 F. The unliquefied portion is, in turn, withdrawn fromseparator l l by a pipe l4 and passes through exchanger 6, where it iswarmed to about 210 F. in further cooling the incoming compressedmixture. Part of the latent refrigeration in this warmed nitrogen ismade available by passing the gas through an expander l6, where it isexpanded with performance of external work to a lower intermediatepressure of about 45' p. s. i. a. and is thereby cooled to a lowertemperature than before. The partially expanded nitrogen is thenconducted back to liquefier 9, where it acts as a cooling agent ineffecting the partial liquefaction of the nitrogen stream from therectifier, and is thereby rewarmed. Since the rewarmed nitrogen is stillunder pressure, its remaining latent refrigeration is made available bypassing it through a second expander ll, where it is expanded tosubstantially atmospheric pressure with performance of external work,and thereby once more recooled to a low temperature. The recoolednitrogen is conducted back to liquefier 9 as a second cooling agenttherein. After being rewarmed in the liquefier, the temperature of thislow pressure nitrogen is still sufficiently low to permit its being usedas a cooling agent in exchanger 6, and it is conducted to that exchangerby a pipe I8 and 3 there further warmed to a temperature of about -189F. The warmed nitrogen is then conducted to exchanger 2 to effect partof the initial cooling of the entering mixture, after which it isdischarged as waste through a pipe l9 at a temperature of about 46 F.

In rectifying the mixture, some of the methane that is separatedaccumulates as a liquid at the bottom of the column around the reboiler3. The warmer mixture entering the reboiler from exchanger 2 causes thisliquid methane to vaporize, forming the vapor reflux necessary forrectification. The gaseous methane product that is sepa-- rated from themixture is withdrawn from the column through a pipe 2| at a temperatureof around l9'7 F. and is conducted to exchanger 2, where it helps tocool the incoming mixture. At the warm end of that exchanger, themethane product is discharged through a pipe 22 at a temperature ofabout 46 F. and at a pressure of about 125 p. s. i. a. and may then bedelivered to gas mains or to storage, as desired, with little or noadditional compression.

The embodiment of this invention shown in Fig. 1 ismost suitable wherethe natural gas to be separated has a fairly high proportion ofnitrogen, on the order of 40 per cent or higher. If the nitrogen contentis lower, for example, around 30 per cent, there will not be sufficientnitrogen available in the open type cycle just described to produce therefrigeration and liquid reflux necessary for rigorous rectification ofthe mixture, so that a substantial proportion of the methane in themixture would be carried off by the waste nitrogen. This difficulty isavoided by using the modified apparatus shown in Figs. 2 and 3, in whichsome of the separated nitrogen is recycled to provide, in effect, aclosed external refrigeration cycle.

Referring to Fig. 2, in which apparatus similar to that in Fig. 1 issimilarly numbered, the entering mixture, here assumed to consist of 30per cent nitrogen and '10 per cent methane, is compressed, cooled,throttled and rectified in the same apparatus and in the same sequenceas in Fig. l. The rectification, however, is preferably carried out at aslightly higher pressure (about 160 p. s. i. a.), and at acorrespondingly higher temperature, than in Fig. 1. Again the separatednitrogen is cooled and partially liquefied by expanding the unliquefiedportion in two stages and utilizing the refrigeration obtained from eachexpansion. The supply of cold nitrogen available for this purpose ishere not limited (as it is in Fig. 1) to the vapor withdrawn fromseparator II, but may be augmented as desired by recycling a portion ofthe nitrogen leaving the warm end of exchanger 6. The recycled nitrogenis conducted through a pipe 3| communicating with the warm nitrogenoutlet of that exchanger to a heat exchanger 32, where it is warmed. Itis then compressed in a compressor 33 to a pressure higher than therectification pressure in column 4, and recooled in exchanger 32 bythenitrogen from pipe 3|. It is further cooled to the temperature of theunliquefied nitrogen withdrawn from separator H by expanding it withperformance of external work in an expander 34 to a pressure of about160 p. s. i. a. This expanded recycled nitrogen is then added by a pipe36 to the supply of cold nitrogen at substantially the same temperatureand pressure withdrawn from the separator by pipe 31. As a result ofthis increased supply of cold nitrogen under pressure, the availablerefrigeration for producing liquid reflux by successive expansions ofnitrogen in expanders l8 and I1 is increased sufficiently to assureeflicient rectification of the mixture to be separated.

In Fig. 3, the apparatus is designed, like that in Fig. 2, to providesufficient liquid reflux for the separation of natural gas mixturescontaining relatively low proportions of nitrogen, for example, aroundthirty per cent or lower. The arrangement differs from that shown inFig. 2 in that the nitrogen that is expanded in expander l3 and thenwarmed in liquefier 9 is still further warmed before it is againexpanded and reintroduced into the liquefier. Another difference is thatthe nitrogen to be recycled is withdrawn from the system at a lowertemperature than is the case in Fig. 2, so that it need not be expandedbefore it is added to the cold nitrogen withdrawn from the separatorand, accordingly, need not be compressed to as high a pressure as inFig. 2.

Referring to Fig. 3 in detail, the entering mixture is cooled,throttled, and rectified as previously described in conection with Fig.2. The separated nitrogen, at a pressure of about 160 p. s. i. a., iswithdrawn from the column, cooled, and partially liquefied, as before.The unliquefled portion is then partly warmed in exchanger 6 andexpanded to a lower intermediate pressure in expander l6, and theresulting refrigeration given up in liquefier 9, as before. Thisnitrogen after being warmed in that liquefier, instead of being againimmediately expanded as in Fig. 2, is first led by a pipe 4| back toexchanger 6, where it is further warmed; and it is then expanded in anexpander 42 (equivalent to expander 11 in Fig. 2) to substantiallyatmospheric pressure to recool it to a low temperature. Part of thistwiceexpanded cold nitrogen is successively used as a cooling agent inliquefier 9, exchanger 6, and exchanger 2. The remaining nitrogen fromexpander 42 is led by a pipe 43 to exchanger 32, where it is warmed. Itis then compressed in compressor 33 to about p. s. i. a. and cooled inexchanger 32 to substantially the same temperature as the unliquefiednitrogen withdrawn from separator Those two nitrogen streams atsubstantially the same pressure and temperature are then commingled inpipe 44 to augment the supply of cold nitrogen in the system.

It is a feature of this invention that the nitrogen that is separatedfrom the mixture under pressure is expanded in a plurality of successivestages to obtain the refrigeration over a specific low temperature rangethat is required to condense the needed liquid reflux for efficientrectification. By first expanding the nitrogen over only a portion ofthe total pressure drop available and using the resultinglow temperaturerefrigeration up to a certain temperature level and then furtherexpanding the nitrogen down to substantially atmospheric pressure, it ispossible to obtain refrigeration within the desired low temperaturerange. These successive expansions may ta e place in more than twostages, with a resultant increase in the low temperature rangerefrigeration; but two expansions, as herein described, are likely to bethe maximum number that will be economically justified in most cases.

Another advantage of this invention is the large temperature differenceprevailing at the warm end of exchanger 2 between the entering mixtureand the two separation products, reflecting an excess of refrigerationat the higher temperature ranges prevailing at the warm end of thesystem. That excess refrigeration can be utilized to repermit the use ofa smaller exchanger 2, with less heat exchange surface, than would bepossible if the temperature differences at the warm end of thatexchanger were smaller.

It is a further feature of this invention that the gaseous methaneproduct recovered at the warm end of exchanger 2 is under a substantialpressure of around 135 to 155 p. s. i a., so that it can be introducedinto delivery mains or storage facilities (where comparable or higherpressures prevail) with little or no additional compression. The methanethat is recovered is approximately 9! percent pure and the yield ofmethane israbout 90 percent, a high figure in view of the simplicity ofthe apparatus used in practicing the invention.

According to the provisions of the patent statues. I have explained theprinciple of my invention and. have illustrated and described what I nowconsider to represent its best embodiment.

However, I desire to have it understood that,

liquefying the nitrogen resulting from said recti-v fication to formliquid reflux for use in rectifying the mixture, partially warming theunliquefied portion of the cooled nitrogen by using it as a coolingagent in cooling the compressed mixture, expanding the nitrogen sowarmed to a lower intermediate pressure with performance of externalwork to recool it to a lower temperature than before, using the nitrogenso expanded as a cooling agent in effecting said cooling and partialliquefaction of nitrogen, again expanding the once-expanded nitrogen tosubstantially atmospheric pressure with performance of external work torecoil it to a low temperature, using the twice-expanded nitrogen sorecooled as a further cooling agent in effecting said cooling andpartial liquefaction of nitrogen, thereafter using said twice-expandednitrogento cool the compressed mixture, and utilizing the separatedmethane at the intermediate rectification pressure to cool thecompressed mixture.

2. Apparatus for separating natural gas mix tures composed substantiallyof nitrogen and methane that includes the following elements: acompressor for compressing the mixture to a high pressure, a first heatexchanger for initially cooling the compressed mixture, a second heatexchanger for further cooling the compressed mixture; a throttle valvefor expanding the cooled compressed mixture to an intermediate pressureand thereby cooling it still further, a

"rectifying column for receiving the throttled mixture and rectifying itat said intermediate pressure to separate its contained nitrogen andmethane, a liquefler for cooling and partially liquefylng the nitrogenresulting from said rectiflcation to form liquid reflux for use inrectifying the mixture, a conduit for conducting the unliquefied portionof the cooled nitrogen to the second heat exchanger as a cooling agenttherein, a first expander for receiving the unliquefied nitrogen warmedin the second exchanger and for expanding it with performance ofexternal work to a lower intermediate pressure to recool it to a lowertemperature than before, a conduit for conducting the nitrogen soexpanded to the liquefler as a cooling agent therein, a second expanderfor receiving the once-expanded nitrogen warmed in the ilquefier andfurther expanding it to substantially atmospheric pressure withperformance of external work to recool it to a low temperature, aconduit for conducting the twice-expanded nitrogen from thesecondexpander to the liquefler as a cooling agent therein, conduits forsuccessively conducting the twiceexpanded nitrogen warmed in theliquefier to the second exchanger and then to the first exchanger as acooling agent therein, and a conduitfor conducting the separated gaseousmethane at an intermediate pressure from the column to the first heatexchanger as a cooling agent therein.

- 3. Apparatus according to claim 2 that includes the followingadditional elements: a third heat exchanger for receiving and furtherwarming a portion of the twice-expanded nitrogen warmed in the secondexchanger, a compressor for compressing the nitrogen so warmed, aconduit for conductin the compressed nitrogen back to the third heatexchanger where it is cooled by giving up heat to and warming thetwice-expanded nitrogen therein, a third expander for receiving thecooled compressed nitrogen and for expanding it to an intermediatepressure with performance of external work to cool it further, and aconduit for adding the nitrogen s0 expanded to the unliquefied portionof cooled nitrogen to augment the supply of the latter.

4. Apparatus for separating natural gas mixtures composed substantiallyof nitrogen and methane that includes the following elements: acompressor for compressing the mixture to a high pressure, a first heatexchanger for initially cooling the compressed mixture, a second heatexchanger for further coolin the compres ed mixture, a throttle valvefor expanding the cooled compressed mixture to an intermediate pressureand thereby cooling it still further, a rectifying column for receivingthe throttled mixture and rectifying it at said intermediate pressure toseparate its contained nitrogen and methane, a liquefier for cooling andpartially liquefying the 4 nitrogen resultin from said rectification toform liquid reflux for use in rectifying the mixture, a conduit forconducting the unliquefied portion of the cooled nitrogen to the secondheat exchanger as a cooling agent therein, a first expander forreceiving said unliquefied nitrogen warmed in the second exchanger andfor expandin it with performance of external work to a lowerintermediate pressure to recool it to a lower temperature than before, aconduit for conducting the expanded nitrogen from the first expander tothe liquefier as a cooling agent therein, a conduit for conducting theexpanded nitrogen warmed in the liquefier back to the second exchangeras a cooling agent therein, a second expander for receiving theonce-expanded nitrogen warmed in the second exchanger and for expandingit further to substantially atmospheric pres- 7 sure with performance ofexternal work to recool it to a low temperature, a conduit forconducting a portion of the low pressure nitrogen leaving thesecondexpander to a third heat exchanger as a coolin agent therein, acompressor for receiving and compressing the low pressure nitrogenwarmed in the third exchanger, a conduit for conducting the compressednitrogen back to the third exchanger where it is cooled by the coolingagent therein, a conduit for addin the compressed nitrogen so cooled tothe unliquefied portion of cooled nitrogen to augment the supply of thelatter, a conduit for'conducting the remainder of the low pressurenitrogen leaving the second expander to the liquefier as a cooling agenttherein, conduits for successively conductin the low pressure nitrogenwarmed in the liquefier to the second heat exchanger and then to thefirst heat exchanger as a cooling agent therein, and a conduit forconducting separated gaseous methane at an intermediate pressure fromthe column to the first heat exchanger as a coolin agent therein.

5. The method of separating as mixtures composed substantially ofnitrogen and methane that includes the following steps: compressing themixture to a high pressure, cooling the compressed. mixture to a lowtemperature, throttling the mixture to an intermediate pressure to coolit further; rectifying the throttled mixture at that intermediatepressure to separate its contained nitrogen and methane, cooling andpartially liquefying nitrogen resulting from said rectification to formliquid refiux' for use in rectifying the mixture, expanding the coolednitrogen that-isnot liquefied in a plurality of stages to substantiallyatmospheric pressure with the performance of external work to recool itto a lower temperature than before, using expanded nitrogen after eachof said expansions to effect said cooling and partial liquefaction ofthe nitrogen resulting from said rectification, compressing some of thenitrogen that has been expanded to substantially atmospheric pressure,cooling the nitrogen so compressed by heat exchange with some of thecold expand ed nitrogen, and addin the cooled compressed nitrogen tosaid unliquefied portion of the cooled nitrogen before the latter isfirst expanded thereb to augment the supply of cold nitrogen in thesystem.

6. A method according to claim 5, in which said cooled compressednitrogen is expanded to said intermediate pressure before it is added tosaid unliquefied portion of the cooled nitrogen.

7. In the separation of natural gas mixtures composed substantially ofnitrogen and methane by rectification under pressure at low temperature,the method of obtaining suflicient liquid reflux to assure efilcientrectification that includes the following steps: cooling andparticularly liquifying the nitrogen fraction resulting from saidrectification, separating the resulting liquid nitrogen from theunliquefied nitrogen, expanding the unliquefied nitrogen in a pluralityof stages to substantially atmospheric pressure with performance ofexternal work to recool it to a lower temperature than before, usingrecooled nitrogen resulting from each of said expansions to effect saidcooling and partial liquefaction of the nitrogen resulting from saidrectification, compressing some of the low pressure nitrogen resultingfrom the last of said expansions, cooling said compressed nitrogen byheat exchange with said low pressure nitrogen before the latter iscompressed, adding said cooled compressed nitrogen to the supply of saidunliquefied nitrogen before the latter is first expanded, and using theseparated liquid nitrogen resulting from said partial liquefaction asliquid reflux in the rectification of the mixture.

8. Apparatus for producing liquid nitrogen reflux for separating naturalgas mixtures composed substantially of notrogen and methane thatincludes the following elements: a rectifying column for receiving themixture after it has been cooled to a. low temperature and for rectify-.ing it at an intermediate pressure to separate its contained nitrogenand methane, a liquefier for cooling and partially liquefying thenitrogen resulting from said rectification to form liquid refiux for usein retifying the mixture, a first ex- ,pander for receiving theunliquefied portion of the cooled nitrogen and for expanding it to alower intermediate pressure with performance of external work to cool itto a lower temperature than before, a conduit for conducting theexpanded nitrogen to the liquefier as a cooling agent therein, a secondexpander for receiving the onceexpanded nitrogen from the liquefier andexpanding it further to substantially atmospheric pressure withperformance of external work to recool it to a low temperature, aconduit for conducting the expanded low pressure nitrogen from thesecond expander to the liquefier as a cooling agent therein, a heatexchanger for receiving and further warming 9. portion of thetwice-expanded low pressure nitrogen that has been used as a coolingagent in the liquefier, a compressor for compressing the nitrogen sowarmed, a conduit for conducting the compressed nitrogen back to theheat exchanger where it is cooled by giving up heat to and warming saidlow pressure nitrogen fiowing through that exchanger, and a conduit forwithdrawing the cooled compressed nitrogen from the exchanger and addingit to said unliquefied port-ion of cooled nitrogen admitted to the firstexpander thereby to augment the supply of cold nitrogen available forexpansion.

9. Apparatus according to claim 8, that includes the followingadditional element: a third expander for receiving the cooled compressednitrogen and for expanding it to said intermediate pressure to cool itfurther before it is added to the unliquefied portion of coolednitrogen.

JOE L. COST.

REFERENCES CITED The following references are of record in the file ofthis patent:

4 UNITED STATES PATENTS Etienne Dec. 19, 1950

1. THE METHOD OF SEPARATING NATURAL GAS MIXTURES COMPOSED SUBSTANTIALLYOF NITROGEN AND METHANE AND OF RECOVERING GASEOUS METHANE THEREFROMUNDER SUBSTANTIAL PRESSURE THAT INCLUDES THE FOLLOWING STEPS:COMPRESSING THE MIXTURE TO A HIGH PRESSURE, COOLING THE COMPRESSEDMIXTURE TO A LOW TEMPERATURE, THROTTLING THE MIXTURE TO AN INTERMEDIATEPRESSURE TO COOL IT FURTHER, RECTIFYING THE THROTTLED MIXTURE AT THATINTERMEDIATE PRESSURE TO SEPARATE ITS CONTAINED NITROGEN AND METHANE,COOLING AND PARTIALLY LIQUEFYING THE NITROGEN RESULTING FROM SAIDRECTIFICATION TO FORM LIQUID REFLUX FOR USE IN RECTIFYING THE MIXTURE,PARTIALLY WARMING THE UNLIQUEFIED PORTION OF THE COOLED NITROGEN BYUSING IT AS A COOLING AGENT IN COOLING THE COMPRESSED MIXTURE, EXPANDINGTHE NITROGEN SO WARMED TO A LOWER INTERMEDIATE PRESSURE WITH PERFORMANCEOF EXTERNAL WORK TO RECOOL IT TO A LOWER TEMPERATURE THAN BEFORE, USINGTHE NITROGEN SO EXPANDED AS A COOLING AGENT IN EFFECTING SAID COOLINGAND PARTIAL LIQUEFACTION OF NITROGEN, AGAIN EXPANDING THE ONCE-EXPANDEDNITROGEN TO SUBSTANTIALLY ATMOSPHERIC PRESSURE WITH PERFORMANCE OFEXTERNAL WORK TO RECOIL IT TO A LOW TEMPERATURE, USING THETWICE-EXPANDED NITROGEN SO RECOOLED AS A FURTHER COOLING AGENT INEFFECTING SAID COOLING AND PARTIAL LIQUEFACTION OF NITROGEN, THEREAFTERUSING SAID TWICE-EXPANDED NITROGEN TO COOL THE COMPRESSED MIXTURE, ANDUTILIZING THE SEPARATED METHANE AT THE INTERMEDIATE RECTIFICATIONPRESSURE TO COOL THE COMPRESSED MIXTURE.